Tuesday, October 9, 2012

For some reason I am in the mood to read a biogeography paper. I didn’t really have a particular topic in mind when I started looking, just a few journals I occasionally peruse. Then I came across a paper about palaeodiversity and the distribution of dinosaurs. I like dinosaurs and I like biogeography. So this must be a win-win. I haven’t visited the idea of dinosaur community structure and distribution since my Dino Eco post back in 2010. That paper concluded that the entire Western Interior of North America may have once been populated by a single dinosaur community with low beta diversity. Today’s paper looks at dinosaur diversity on a global scale. Last month, authors Philip Mannion et al. published a paper in Global Ecology and Biogeography that used the dinosaur fossil record to examine spatial patterns in terrestrial biodiversity.

To really get into this paper we’ll have to first explore the latitudinal biodiversity gradient (LBG). The LBG is essentially a biodiversity pattern in which species richness (a simple count of species) is highest in the tropics and declines polewards. It is a well-recognized pattern with causes that are a little less clear and that are still strongly debated. The two strongest hypotheses are that climate is the prime driver that influences biodiversity directly or via increased productivity in the tropics (warm areas are more hospitable and produce more food) or that the global distribution of area where the greater land area supports more species (the more land you have the more species will fit on it). The fossil record offers a deep time perspective that may help figure out the causes of the LBG we see today. The authors of this study chose a group of animals that was widespread, ecologically diverse, well studied, belonged to a long-lived clade, and had a fossil record that was adequate for the application of sampling standardization techniques. This gave them dinosaurs of Mesozoic terrestrial ecosystems for 160 million years, from the Late Triassic to the terminal Cretaceous (230-65.5 Ma).

Mannion and his colleagues decided to use genera as the taxonomic unit of analysis for their study as the species scale can be inconsistent in its taxonomic treatment and the family scale tends to have arbitrary content (some families comprise a single genus while others are diverse). They then assembled a comprehensive dataset of Mesozoic dinosaur genera (738), including birds. Using the Paleobiology Database, they compiled stratigraphic ranges and modern geographic coordinates for occurrences, converting modern day coordinates to paleolatitudes using software called PointTracker (uses palaeogographical reconstructions of continental drift to transform present-day coordinates to past ones). They used a number of different methods to account for sampling biases in the fossil record, and they analyzed the data in seven time slices representing epochs and then the entire Jurassic and cretaceous periods as bins.

The authors found that dinosaurs did not conform to the modern LBG. Dinosaur diversity was found to be highest at temperate latitudes rather than tropic. This result was consistent across the different time slices suggesting that the pattern was not controlled by climate fluctuations; in fact, it is possible that the Mesozoic climate gradient was much weaker than it is today. Rather, or because of this weakness, the driver of this diversity was likely the result of greater land area in these latitudinal belts. These larger land areas may also have facilitated the gigantism attained by many dinosaur species.

Unfortunately, these results suggest modern diversity patterns cannot be extrapolated into deep time. Guess we'll have to look somewhere else. However, it does add support to the hypothesis that land area is the primary control on the terrestrial LBG during times of weakened climatic gradient. And that, on its own, is a really interesting conclusion. The contrary, modern vs. past result also indicates that there may have been some kind of shift that took place during the middle Cenozoic that gave rise to the diversity patterns that we see today. I think I see a future study!

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